How to merge textures into single image
Look up Blender Texture Atlas.

also,
You can create a new image, in the image editor, large enough to hold the resolution you want. 
Then, create a new texture layer, to use the new image, and bake the original textures to the new Texture. You will just have to move the UV maps around so they don't overlap during baking.

Info on that here and here.